Current limiting fuses have been employed in power networks for many years and the basic technology is fairly old. However, due to the large ratio, approximately two orders of magnitude, of limited current to nominal current, their use is restricted to low values of nominal currents, typically under 200 Amperes.
One of the most basic designs of a passive current limiting fuse requires a multitude of arcs to be created after the melting of series of constrictions along a thin ribbon of metal such as silver. This multitude of arcs withstands the voltage of the network in which the fuse is inserted and auto-extinguishes in a fraction of a cycle.
This auto-extinction is usually achieved by embedding the element in a highly compacted sand or quartz powder.
In normal operating conditions, the melting point of constrictions, as the AC current is very slowly increased, determines the maximum nominal operating current. In fault conditions however, the current rises very quickly above the nominal value and when it is high enough to have provided sufficient energy to heat the constrictions to a high temperature, it will cause them to melt. This increases their resistance and initiates the series of arcs that complete the action of the fuse.
Advantages are that such fuses are fail safe, silent and passive, no external triggering is required. The main disadvantage is that the resistivity of the metals used, for instance silver, does not increase rapidly with temperature. This leads to large ratios of peak limited current to maximum nominal current and high let-through energies.
Recent developments, to increase the range of nominal currents for the application of fuses to power networks, have led to active fuses that are triggered by an explosion in a high nominal current element in parallel with a more conventional fuse element that is used to deviate the current from the high nominal current element whilst its explosion and extinction occurs.
Subsequently, this more conventional element operates in the usual manner to completely interrupt the fault current. In this way, the operating maximum nominal current may be increased without duly increasing the peak limiting current and the let-through energy. However, disadvantages are that the action is active, and as non-operation may cause considerable damage, this type of fuse requires a very reliable triggering system. Moreover, often the explosive charge has to be renewed periodically, and the fuse operation may also cause a loud noise.
In view of the above, there is a need for a method and apparatus that will overcome the above-identified drawbacks.